This proposal will study the role of Toll-like receptor (TLR) 3-mediated responses in the control of West Nile (WN) virus infection. WN virus is a mosquito-borne pathogen that has recently emerged in the United States and may cause encephalitis. TLRs are a family of innate immune-recognition receptors that recognize molecular pattems associated with microbial pathogens, and induce antimicrobial immune responses. In two recently published studies we have demonstrated that (a) protective immunity can be elicited in a murine model of WN virus infection and that (b) TLR3 recognized double-stranded RNAs and is therefore likely to be important in host responses to viruses. Moreover, our preliminary data demonstrate that TLR3-deficient (TLR3 -/-) mice are more susceptible to WN virus infection that control animals, further suggesting that TLR3 plays an important role in the control of viral infection in vivo. Our proposal has 3 parts. First (l) we will more clearly define the role of TLR3 in viral recognition because our preliminary data show that mammalian TLR3 recognizes dsRNAs, and that activation of the receptor induces the activation of NF-kappaB and the production of type I interferon (IFNs). Second (2), we will carefully examine WN virus infection in TLR3 # mice. WN virus will serve as our model because (a) this infection is spreading throughout the U.S., is becoming a pubic health concern, and is a catergory B agent for biodefense (b) we have refined a murine model of WN virus infection and have effectively used the model to study immunity against WN virus, and (c) our preliminary studies demonstrate that WN virus infection is more severe in TLR3 -/- mice than in control animals. Thirdly (3), we will attempt modulate WN viral infection in mice by enhancing TLR3-mediated responses. These studies should increase our understanding of the role of TLR3 in viral recognition, and may lead to new strategies to prevent, or treat, WN virus infection. [unreadable] [unreadable]